1. Field of the Invention
The present invention relates, in general, to tracking control systems that may be used in controlling the position of an actuator and a read/write head of a computer mass storage device, such as a disk drive. More particularly, the present invention relates to circuitry and methods for increasing the dynamic range of a signal used in the tracking system to control the read/write head actuator of the disk drive.
2. Description of the Related Art
Most computer systems typically include one or more disk drives for storing and retrieving data. In general, they include a randomly accessible rotating storage medium (e.g., disk platters) on which data is encoded. Typically, most disk drives will have several disks that are spanned by an equivalent number of read/write heads that are grouped together and move as a single unit.
The read/write head is mounted on an actuator arm that is attached to a voice coil motor (VCM) capable of moving the read/write head assembly across the disk surface at a high speed to the desired data track. Controlled by the track control or servo system, the voice coil motor will "seek" to the selected track and when the head reaches the selected track, the control system performs a "track following" function that positions and maintains the read/write head over a centerline of the selected track.
The seek and track following functions impose different constraints on the servo or track control system. During a seek operation, the actuator must move as fast as possible to minimize the time required to get the read/write head to the approximate location where the desired track lies. Thus, velocity, trajectory and arrival characteristics of the actuator are the determining control factors for this function. During the track following function, on the other hand, it is the accuracy with which the read/write head can be made to follow the centerline of the data track that is important.
FIG. 1 is a block diagram illustrating a typical track control system used in a disk drive to position an actuator/head over the desired data track. As shown, the track control system 100 includes a servo controller 110, a power amplifier 128, a position demodulator 130, and a tracking actuator 132. The servo controller 110 includes a processor and a compensator circuit or code 112 that generates a 12-bit command word in response to a request from the processor. The command word is 12 bits in this example but could be other widths. The 12-bit command word is sent to a digital-to-analog converter 120. The digital-to-analog converter 120 converts the command to its analog equivalent or control voltage Vc, and transmits it to the power amplifier 128. The magnitude of the voltage supplied to the power amplifier 128 determines whether the actuator will perform a "seek" or a "track-follow." This is due to the fact that the voltage required to initially accelerate or move the actuator to the appropriate data track is greater than the voltage required to keep the actuator over the centerline of the data track. Once received, the power amplifier 128 amplifies the voltage Vc and generates a proportional current output Im that is transmitted to the tracking actuator 132. The proportional current output Im therefore causes a proportional acceleration or movement of the actuator.
FIG. 1 also shows the feedback mechanism used to adjust and maintain the position of the read/write head over the data track. Control information embedded in the data provides inter-track positioning information so that a head positioning error, indicative of the difference between the estimated head position and the desired head position is sent from tracking actuator 132 to the position demodulator 130. The position demodulator 132 extracts both a positioning error voltage Vp at 124 and a track number Np at 126 from the head positioning error. The positioning error Vp 124 is converted into digital data by the analog-to digital-converter 122. Once converted, the positioning error Vp and the data track number Np are processed by the appropriate scaling gains 116, 118 and summed together at 114 and forwarded to the compensator/processor circuit 112. This scaling and summing is typically done in the compensator code by the processor. FIG. 1 shows how the scaling and summing functions would be performed within a dotted line 113 to the compensator/processor circuit 112. In response to the head position error, compensator/processor circuit 112 adjusts the control voltage to either move the actuator to another data track or adjust the read/write head to the proper position over the centerline of the desired data track.
In response to high consumer demand for higher performing hard drives, drive manufacturers have been building disk drives with an increased number of tracks which are laid-out with an increased level of density. Unfortunately for designers of tracking control systems, this places an increased demand for speed during seek operations as well as more precise track following operations. As a result, the ability of a control system to efficiently move the read/write head to the appropriate track and maintain the read/write head over the centerline of the data track is becoming more difficult.
One approach that may be taken to improve performance is to increase the dynamic range of the control voltage Vc produced by the digital-to-analog converter 120. FIGS. 2a through 2c illustrate several conventional approaches which may be attempted to increase the dynamic range. FIG. 2a illustrates a technique that implements a digital-to-analog converter 202 that is able to accommodate a processor command with a greater number of control bits (i.e., N+m Bits). By doing this, the range or swing of the control voltage Vc is increased while minimizing the errors normally found in signals generated by digital-to-analog converters used in this environment. The problem with this approach is that it does not overcome the system noise problems inherent with generating control voltages with greater dynamic range. In addition, the die size of the digital-to-analog converter typically doubles for every bit added to the command word, thus, increasing the size and the cost of such a component.
FIG. 2b shows another conventional technique that can be used to increase the dynamic range of the control voltage Vc generated by the digital-to-analog converter 120. As shown, an attenuator 204 and an associated reference voltage 206 (DAC Ref. Voltage) controls the input of the digital-to-analog converter 120. Depending on the desired positioning function (i.e., seek or track-follow), the digital-to-analog converter 120 will receive either a high or low level reference signal from the attenuator 204. In this technique therefore, the voltage reference that sets the signal swing is modified by the attenuator 204, which is a switchable attenuator. Thus, the digital-to-analog converter 120 will produce the appropriate range or swing of the control voltage Vc. This implementation does allow designers to reduce the die area of the tracking control system, but also has the downside of increasing digital-to-analog conversion errors. Such errors include, for example, a substantial increase in linearity errors that are introduced by the digital-to-analog converter 120 when in the track-following mode. This increase in linearity errors will therefore cause a substantial decrease in control during track-following operations. Unfortunately, it is during track-following that the greatest level of control and accuracy is needed in order to ensure that the read/write head is well positioned over the centerline of a given track. Further yet, the technique of FIG. 2b does not address the issues of system noise.
FIG. 2c illustrates another technique that may be used for increasing the dynamic range of the control signal Vc. Here, the reference voltage 206 is supplied directly to the digital to analog converter 120. This keeps dynamic range or swing of the control voltage Vc constant. To modify the constant Vc, a variable gain amplifier 210 is implemented to shift between the seek mode and the track-follow mode. Unfortunately, this technique also fails to address the problem of noise pickup or offsets.
In further attempts to increase the dynamic range of the control voltage Vc, designers may integrate additional components, such as attenuators and switches, directly on the printed circuit board (PCB) that is also configured to receive the servo controller 110 in the form of an integrated circuit (IC) chip. Some of these external components will therefore include active devices, such as field effect transistors (FETs) or FET switch ICs. As is well known, when active devices are integrated directly onto the PCB, an added level of cost and complexity is introduced into the design. More specifically, this external integration of active components can have the unfortunate downside of increasing the cost of the entire tracking control system because the PCB components are known to significantly add to the cost of an IC solution, and increased engineering design is typically needed to ensure that the IC chip properly interfaces with the PCB components.
In view of the foregoing, there is a need for circuitry and methods that enable a wider dynamic range control signal between a digital-to-analog converter and a power amplifier within a disk drive tracking control system. There is also a need for circuitry and methods that enable the increase in dynamic range without increasing the die size of the circuitry and without requiring external PCB active components. In addition, a needed also exists for circuitry and methods that provide for an increased dynamic range in the generated control signal without increasing the control signal's susceptibility to noise pickup or offsets.